1. Field of the Invention
The present invention relates to a method for growing single crystals of barium titanate [BaTiO3] and barium titanate solid solutions [(BaxM1xe2x88x92x)(TiyN1xe2x88x92y)O3] and, more particularly, to a method for growing single crystals based on the principle that a secondary abnormal grain growth occurs at an elevated temperature higher than a predetermined temperature during the heat treatment of polycrystalline barium titanate and that the predetermined temperature depends on various additives used, whereby only a few secondary abnormal grains can continuously grow to produce single crystals by controlling the heat treatment temperature of a polycrystal or locally adding an additive to a sample.
The term xe2x80x9cbarium titanate solid solutionxe2x80x9d as used herein refers to a solid solution containing a solute such as MO or NO in barium titanate, and has a chemical formula of (BaxM1xe2x88x92x)(TiyN1xe2x88x92y)O3 which is distinct in physical and chemical properties from a mixture of pure barium titanate powder with an impurity.
The invention also relates to a method for producing single crystals of barium titanate and barium titanate solid solutions in which the single crystal of barium titanate [BaTiO3] is embedded as a seed single crystal in the polycrystal of the barium titanate and barium titanate solid solutions, followed by heat treatment, thereby allowing a single crystal to continuously grow in the polycrystals by a secondary abnormal grain growth at an interface between the polycrystal and seed single crystal, wherein the single crystals of barium titanate and barium titanate solid solution thus obtained have the same composition as that of the original polycrystal as well as the same structure as that of the seed single crystal. The invention further refers to an economical method for producing a large amount of single crystals by embedding the produced single crystals of barium titanate and barium titanate solid solution prepared by the above-stated method as a seed single crystal in the polycrystal.
Still further, the present invention relates to a method for producing single crystals of barium titanate and barium titanate solid solutions, comprising the steps of: implanting a seed single crystal in the polycrystal of barium titanate while restraining abnormal grain growths by performing a heat treatment under a reductive atmosphere, and then controlling the temperature, composition and atmosphere of the heat treatment to cause an abnormal grain growth at an interface between the polycrystal and the seed single crystal, thereby allowing the seed single crystal to continuously grow into the polycrystal.
2. Description of the Related Art
The single crystals of barium titanate and barium titanate solid solutions are widely used as a material for optical devices such as piezoelectric sensors, optical valves, optical interrupters, and phase-matching mirrors, etc. and are considered to be promising substrate materials for various thin film elements.
The conventional method for growing a single crystal of barium titanate [BaTiO3] requires expensive facilities and involves many problems in producing a large amount of large single crystals because of an extremely complicated process for growing the single crystals.
Grain growth takes place during the step of sintering polycrystals, in which case only a few grains are sometimes rapidly grown in an abnormal manner relative to the most normal grains. It is appreciated that controlling the growth of such a few abnormal grains in polycrystals may allow single crystals to be easily produced without a melting process.
A general method for single crystal growth using the melting process is called liquid-state single crystal growth (LSCG), and a method for single crystal growth by heat treatment of polycrystals is referred to as solid-state single crystal growth (SSCG). The SSCG method has been suggested since the 1950""s and demonstrated as an effective method of preparing single crystals of a metal, which is limited to only a few types. It is however reported that it is difficult to prepare single crystals large enough for practical use from an oxide by this method, because the oxide is slow in grain growth and it is difficult to control the nucleation of abnormal grains.
Since the emergence of a flux method for single crystal growth of barium titanate and barium titanate solid solution, subsequent methods for single crystal growth of barium titanate have been developed, such as the zone melting method and the top-seeded solution growth (TSSG) method. However, these methods present some problems in that the crystals thus obtained do not have a cubic structure, as normally desired, but a hexagonal structure which is stable at high temperature and that the methods require considerably complicated production techniques and facilities. So, the methods are too problematic for use in preparing a large amount of inexpensive, large-sized single crystals almost without residual stress. The single crystals of barium titanate grown by the flux methods have a thickness of less than 1 mm and a diameter of several millimeters and thus are actually restrained in practical uses. It is known that the TSSG method, which has the advantages of the flux method and the Czochralski method, is applicable to the growth of relatively large single crystals of barium titanate almost without residual stress. However, the TSSG method also requires complicated facilities and sophisticated skills and is inadequate as a method for preparing a large amount of single crystals at a low cost.
Meanwhile, there has been made an attempt to obtain single crystals by subjecting polycrystals of ferrite, barium titanate [BaTiO3] and aluminum oxide [Al2O3] to sintering. This method for single crystal growth involves sintering a powder impregnated with single crystals as seed single crystals or providing an interface between the sintered bodies and the seed single crystals, followed by heat treatment. Disadvantageously, this method is not suitable to preparing single crystals large enough for practical uses because the growth of single crystals is retarded relative to the conventional growth methods.
For single crystal growth of barium titanate [BaTiO3], there is reported a method for preparing single crystals by adding particles having a (111) twin plane or a seed forming agent to form a (111) twin plane. However, this method has a problem in that the additives are alloyed in barium titanate during the heat treatment and thus pure single crystals of barium titanate cannot be produced. Also, the method has a second problem in that it is most important but difficult to limit the number of crystals to be grown abnormally and that the single crystals thus obtained are small in size.
Contrarily, the invention provides a method for single crystal growth by controlling the temperature for heat treatment or adding seed single crystals and additives so as to control the nucleation and growth of the secondary abnormal grains in performing a heat treatment on barium titanate and barium titanate solid solution. The present invention makes it possible to prepare a large amount of the single crystals of barium titanate and barium titanate solid solution without expensive facilities or complicated processes.
Alternatively, the present invention involves sintering barium titanate and barium titanate solid solution under reductive atmosphere to restrain abnormal grain growth and obtain dense polycrystals, implanting seed single crystals on the polycrystals, and performing a heat treatment under the condition such that the abnormal grain growth occurs only at an interface between polycrystals and seed single crystals so as to grow the seed single crystals into the polycrystals successively, thereby preparing single crystals of barium titanate and barium titanate solid solution.
It is, therefore, an object of the present invention is to overcome the problems of the conventional single crystal growth (i.e., liquid-state single crystal growth) method requiring a melting process, and to provide a method for growing single crystals of barium titanate and barium titanate solid solutions through a general heat treatment without special equipment, thereby producing a large amount of the single crystals at reduced cost.
To achieve the object of the present invention, there is provided a method for growing single crystals of barium titanate [BaTiO3] and barium titanate solid solutions [(BaxM1xe2x88x92x)(TiyN1xe2x88x92y)O3], which shows primary and secondary abnormal grain growths by means of a temperature condition higher than a predetermined temperature, or the composition of a polycrystal and an atmosphere of heat treatment, comprising the steps of: (axe2x80x2) performing a heat treatment at a temperature higher than a secondary abnormal grain growth activating temperature to produce only a few secondary abnormal grains; and (a) performing a second heat treatment at a temperature lower than the secondary abnormal grain growth activating temperature of the polycrystal, thereby allowing only a few secondary abnormal grains to grow continuously.
In another aspect of the present invention, there is provided a method for growing single crystals of barium titanate [BaTiO3] and barium titanate solid solutions [(BaxM1xe2x88x92x)(TiyN1xe2x88x92y)O3], which shows primary and secondary abnormal grain growths at a temperature higher than a predetermined temperature, or depending on the composition of a polycrystal and an atmosphere of heat treatment, comprising the steps of: (axe2x80x3) adjoining a seed single crystal to the polycrystal; and (a) performing a second heat treatment to cause the secondary abnormal grain growth at an interface between the polycrystal and the seed single crystal, thereby allowing the same structure of the seed single crystal to grown continuously in the polycrystal.
In still another aspect of the present invention, there is provided a method for growing single crystals of barium titanate [BaTiO3] and barium titanate solid solutions [(BaxM1xe2x88x92x)(TiyN1xe2x88x92y)O3], which shows primary and secondary abnormal grain growths at a temperature higher than a predetermined temperature, or depending on the composition of a polycrystal and an atmosphere of heat treatment, comprising the steps of: (axe2x80x3) preparing a polycrystal of barium titanate inhibited from the abnormal grain growths through sintering under a reductive atmosphere and adjoining the polycrystal to a seed single crystal; and (a) controlling the temperature, composition and atmosphere to cause abnormal grain growths at an interface between the polycrystal and the seed single crystal, thereby allowing the seed single crystal to continuously grow into the polycrystal. In this method, single crystals of barium titanate and an oxide having the same crystallographic structure as barium titanate can be used as a seed single crystal. Such an oxide is SrTiO3 or CaTiO3. Preferably, step (a) is performed under an oxidative atmosphere.